A very well known example in the prior art for measuring dissolved oxygen is the Clark electrode. See, for example, pages 11 and 12 of "Oxygen Measurement by the Electrode Method" (B Hagiwara, published by Kodansha). A gas-permeable film of PTFE is fixed over an end opening of a hollow body containing an electrolyte e.g. a phosphate buffer with KCl. A cathode comprising a platinum wire extending coaxially through a glass or epoxy rod extends through the body to the gas permeable film, the cross-section of the Pt wire at the end of the rod being exposed, polished and contacting the film. An anode, usually Ag/AgCl, also contacts the electrolyte within the body. Oxygen penetrating the PTFE film is reduced at the cathode and a current dependent on the amount of oxygen flows through a circuit connecting cathode and anode under an applied voltage, with concomitant oxidation of Ag to AgCl at the cathode.
The Clark electrode is highly reliable and has a good lifetime e.g. one year. However it is very expensive and difficult to make, since it must be machined.